1. Field of the Invention
The present invention relates to a liquid injection device or the like used for an ink jet printer or the like.
2. Related Art of the Invention
In recent years, ink jet printers have been rapidly becoming popular as a printer capable of implementing color printing at low price. It is an ink injection device that determines the performance of this ink jet printer, and it is a liquid injection device which intermittently injects fine liquid particles.
As a conventional liquid injection device, the description will be made by exemplifying a typical ink injection device for ink jet printers. The ink injection device for ink jet printers can be generally classified under the following two types: heat type and piezo-electric type.
The injection principle of the typical heat type will be described with reference to FIG. 24. FIG. 24 is a cross-sectional view showing an ink injection element constituting an ink injection device. An ink pressurizing chamber 803 is provided in space interposed between a substrate 801 and a substrate 802, and a heater 804 is provided between the ink pressurizing chamber 803 and the substrate 801. Ink is supplied from an ink storage (not shown) provided outside of the ink injection element by a capillary phenomenon or a suction operation from outside, and is supplied to the ink pressurizing chamber 803 through an ink passage 806. When the heater 804 is electrically energized in this state, the ink intensely boils to generate air bubbles. The growth of these air bubbles increases the pressure within the ink pressurizing chamber 803 to inject the ink through an ink injection port 806. An actual ink injection device is configured by a plurality of ink injection elements described above lined.
Next, an example of a typical piezo-electric type ink injection device for ink jet printers will be described with reference to FIG. 25, which is a cross-sectional view for an ink injection element. In FIG. 25, a reference numeral 813 denotes a piezo-electric actuator, which is driven by a piezo-electric operation, and is, for example, a bimorph element configured by two piezo-electric elements, or an unimorph element configured by a piezo-electric element and a diaphragm, or the like. A reference numeral 818 denotes an ink passage; 819, an ink pressurizing chamber; and 820, an ink injection port. A portion indicated by broken lines in the piezo-electric actuator 813 schematically shows deformation of the piezo-electric actuator. Ink is supplied from an ink storage (not shown) provided outside of the ink injection element at the beginning by a capillary phenomenon or a suction operation from outside, and is supplied to the ink pressurizing chamber 819 through the ink passage 818. When the piezo-electric actuator 813 is caused to become deformed as indicated by the broken lines in a state in which the ink passage 818 and the ink pressurizing chamber 819 are filled with the ink, the pressure within the ink pressurizing chamber 819 increases to inject the ink through the ink injection port 820. The actual ink injection device is constructed such that a plurality of elements described above are arranged in a line because of high-speed printing.
In recent years, requests for an ink jet printer capable of expressing more colorful colors at low price have been increasing, and in order to perform more colorful color printing by an ink jet printer using such a liquid injection device, it is necessary to increase a number of gradation levels perunitpicture element (pixel), that is, to implement multi-tone printing.
A method for implementing such multi-tone printing will be described below.
First, a method using an area modulation system is named. This method is to form an unit picture element (pixel) for expressing light and dark density by hitting a plurality of dots without superimposing one on another, and to express the gradation by changing a rate of ink per unit picture element.
Secondly, a method using superposedly dotting is named. This method is to inject ink on the same place a plurality of times and to express the gradation by changing the size of unit pixel.
Thirdly, a method using a density modulation system is named. This method is to express the gradation by using a plurality of inks of different coloring matter density.
Fourthly, a method using a dot modulation system is named. This method is to discharge ink drops having different sizes through the same injection port, and is capable of change the size of dot for each injection.
However, the above-described methods for implementing multi-tone printing have the following problems respectively.
First, problems of the area modulation system will be described. In the area modulation system, since dots to be recorded are thinned out to express light and dark density, the size of the unit pixel required to express the light and dark density becomes large to lower actual recording density. Therefore, when an attempt is made to express multi-tone light and dark density, print with conspicuous surface roughness in a low-density portion is produced. In order to reduce the lowered recording density, the size of the unit pixel is made as small as possible. In other words, it becomes necessary to make the minimum dot diameter smaller. In order to reduce the minimum dot diameter, or to discharge small ink drops, it is necessary to reduce the injection port diameter or to device the discharging method. Since, however, the conventional injection port diameter of the ink jet head is as small as 20 to about 30 microns, in order to bore an injection port with a smaller diameter than the injection port diameter, a more difficult manufacturing process is required and yet more injection ports must be provided, leading to an increase in manufacturing cost. Also, in such smaller injection ports, ink clogging or defective discharging due to dust or the like mixed into the ink is prone to occur, possibly deteriorating the reliability of the products. Also, even if the above-described problem was solved and ink drops having small dot diameter could be injected, a problem that the printing speed becomes slower would occur. This is a problem which occurs when more ink drops must be discharged in order to fill in the same area, and in order to solve this problem, there become necessary devices such as (1) to shorten discharge repeating time (high-speed driving of the ink injection device) and (2) to increase the number of elements (number of injection ports) The former (1) is difficult because of rate-determining of heat transmission time in the case of the heat type ink injection device. In the case of the piezo-electric type ink injection device, it may be driven at high speed as compared with the heat type, but there is a limit because of the trackability of the liquid to the piezo-electric actuator, and the like. Also, the latter (2) causes problems such as complicated device and reduced yields in the manufacture, leading to an increase in cost.
Secondly, as regards the method using superposedly dotting, the generally same problem occurs as the problem in the area modulation system because a multiplicity of smaller dots than the unit pixel must be hit in the unit pixel. Further, since liquid is shot onto the same point intensively in addition, a granulation phenomenon is prone to occur, easily causing print with surface roughness feeling.
Thirdly, the problem of density modulation system will be described. In the density modulation system, it is necessary to have a plurality of inks of different coloring matter density, and there arises a problem that the device becomes complicated and larger, and the cost is increased. Also, with an increase in type of the ink, there arise problems that the number of elements which can be actually used also reduces (number of elements/type of ink) and the printing speed is slowed down.
Fourthly, problems of the dot modulation system will be described. The dot modulation system modulates the dot diameter by directly changing an injected amount to be injected at a time, and therefore, the above-described problem in the three types of modulation systems is greatly reduced. For example, since the unit pixel comprises one dot, light and dark expression can be made without causing any increase in the unit pixel size. Also, since it is not necessary to hit a multiplicity of dots on the unit pixel, it is possible to perform high-speed printing without necessitating any increase in the number of injection ports.
In the foregoing, of the conventional techniques for implementing the multi-tone printing, the fourth dot modulation system is most excellent.
In the conventional ink injection device, however, the dot modulation system itself, that is, to modulate the dot by injecting liquid drops having different diameters through the same liquid injection device is very difficult.
In the conventional heat type ink injection device, since ink is injected using an intense boiling phenomenon, it is very difficult to control, and in a printer using this ink injection device, there is used two-valued control as to whether ink is injected or not injected. Therefore, light and dark expression is normally performed by the area modulation system, or the density modulation system or by a combination of both, and there are such problems as described above.
In the conventional piezo-electric type, it is possible, unlike the heat type, to control an amount of ink to be injected to some degree by controlling the displacement amount of the piezo-electric actuator. As matters now stand, however, the number of gradation levels is as insufficient as two to about six, and the modulation width is as insufficient as about two. The modulation width is, however, defined as a ratio of a minimum amount of ink injected (volume) to a maximum amount (volume) of ink injected. Originally, it should be defined as a ratio of diameters of dots recorded, but it has been defined as described above because the dot diameter greatly changes depending upon the physical properties of the ink and paper. Therefore, even in the conventional piezo-electric type, on expressing the gradation, light and dark expression is normally performed by the area modulation system, or the density modulation system or by a combination of both, and there are such problems as described above.
In the piezo-electric type liquid injection device, conditions on implementing the dot modulation and problems in the existing condition will be further described in detail.
In order to implement the dot modulation, it is first necessary to enlarge the width of energy amount which can be supplied to the liquid to be injected. This is a condition required to make the modulation width large because the necessary energy amount differs depending upon the amount of liquid drops to be injected. Normally, the rate is determined by its upper limit value, but it is necessary to improve the energy utilization efficiency of the device because there is a limit to the amount of energy which can be supplied to the device. Secondly, in order to inject large liquid drops, it is necessary to arrange the structure such that the displacement amount of the piezo-electric actuator can be taken large (large displacement properties). In the injection of comparatively large liquid drops, the larger the displacement amount of the piezo-electric actuator is, the larger the liquid drop diameter becomes. In other words, the liquid drop diameter can be modulated with the displacement amount as a parameter. Thirdly, in order to inject small liquid drops, it is necessary to change the pressure within the liquid pressurizing chamber from low pressure to high pressure in a very short time while reducing the displacement amount of the piezo-electric actuator (response characteristic of pressure). In this case, the liquid drop can be modulated with the rate of change in pressure within the liquid pressurizing chamber as a parameter. Fourthly, it is necessary to accurately control the above-described displacement amount of the piezo-electric actuator and the rate of change in pressure within the liquid pressurizing chamber (controllability).
In the conventional piezo-electric type liquid injection device, it is difficult to satisfy the foregoing conditions. With reference to FIG. 25, the description will be further made. When the piezo-electric actuator 813 is displaced downward (state indicated in broken lines), the pressure within the ink pressurizing chamber 819 increases. With this increase in pressure, the piezo-electric actuator 813 undergoes a reaction from the ink within the ink pressurizing chamber 819 and the displacement amount of the piezo-electric actuator 813 becomes exceedingly small as compared with the displacement amount when there is no load such as ink.
When the piezo-electric actuator 813 is displaced downward (state indicated in broken lines), the pressure within the ink pressurizing chamber 819 increases, and the pressure within the ink passage 818 also increases at the same time. In this case, of work done which has been operated on the ink by the displacement of the piezo-electric actuator 813, that is, work done supplied to the ink, work done used to increase the pressure within the ink pressurizing chamber 819 is one-half or less of the work done. The remaining energy is consumed by the increase in pressure and loss in the ink passage 818. This can be improved to some extent by thinking out the shape of the vicinity of the border between the ink passage 815 and the ink pressurizing chamber 818 so as to solve the problem (for example, a back-flow valve is provided), but this cannot fundamentally solve it, but is limited to improving the energy utilization efficiency by 10 to about 20% at most. Also, since the piezo-electric actuator 811 is normally on all sides fixed to the ink pressurizing chamber 818 to keep the ink pressurizing chamber airtight, the displacement amount is small in principle. Further, when an attempt is made to increase the amount of ink injected to make up for such small displacement amount, it is necessary to enlarge the cross-sectional area of the ink pressurizing chamber 818 having the direction of displacement of the piezo-electric actuator 811 as the direction of the normal thereto. This causes problems that the energy loss in the ink chamber will become greater, and that the above-described reaction will further become greater. For the reasons described above, the present piezo-electric type liquid injection device has a number of gradation levels of 2 to 6, and modulation width of about 8.
As a technique to increase the modulation width with the energy utilization efficiency as it is, it can be considered to take the variable width of the amplitude value of the applied voltage large, that is, to make the maximum applied voltage high. This causes the following problems. Firstly, since the present piezo-electric actuator has insufficient dielectric strength, the element life is deteriorated or the element is broken. Also, when the element is damaged, the piezo-electric actuator normally enters a short-circuited state, causing a problem also in safety. Secondly, the piezo-electric element is often subjected to a polarization process in the driving voltage applied direction, and the application of high voltage causes part of the polarization or the whole to be lost, deteriorates the characteristic properties, or causes the piezo-electric property itself to be lost in extreme cases. Also, the disappearance of the polarization leads to deteriorated element life. Thirdly, in the case of home use, an increase in applied voltage itself causes a problem in safety and cost effectiveness. Fourthly, the surrounding circuit element becomes complicated and larger, thus increasing the cost. For the above-described problems, an increase in applied voltage is not desirable in practical use.
A liquid injection device and an injection method according to the present invention solve the above-described problems, and their object is to improve the energy utilization efficiency of the liquid injection device, or to enable the actuator to be driven in large displacement, or to make the rate of change in pressure within the pressurizing chamber higher.
One aspect of the present invention is
a liquid injection device, comprising:
a liquid pressurizing chamber having one or a plurality of apertures;
a liquid injection port provided at a part of said liquid pressurizing chamber;
a liquid pressurizing member arranged adjacent said liquid pressurizing chamber; and
a liquid passage arranged adjacent said liquid pressurizing chamber,
within said aperture, a peripheral edge portion of said aperture located at a position opposite to said liquid pressurizing member, and said liquid pressurizing member being arranged to be apart from each other at a gap with a predetermined size when said liquid pressurizing member is driving or even at a non-driving time; and
liquid being injected through said liquid injection port by driving said liquid pressurizing member to thereby pressurize said liquid supplied from said liquid passage into said liquid pressurizing chamber.
Another aspect of the present invention is
the liquid injection method in a liquid injection device, comprising:
a liquid pressurizing chamber having one or a plurality of apertures;
a liquid injection port provided at a part of said liquid pressurizing chamber;
a liquid pressurizing member arranged adjacent said liquid pressurizing chamber; and
a liquid passage arranged adjacent said liquid pressurizing chamber, within said aperture, a peripheral edge portion of said aperture located at a position opposite to said liquid pressurizing member, and said liquid pressurizing member being arranged to be apart from each other at a gap with a predetermined size; and
pressurizing the liquid within said liquid pressurizing chamber to inject said liquid through said liquid injection port by driving said liquid pressurizing member to thereby displace said liquid pressurizing member in such a direction as to change said gap between said liquid pressurizing member and the peripheral edge portion of said aperture.
Still another aspect of the present invention is a manufacturing method for an unimorph type piezo-electric actuator configured by a diaphragm and a piezo-electric substrate which are fixed at both ends or at one end, comprising the steps of: bonding said diaphragm, after it is machined into a predetermined shape, to said piezo-electric substrate; polishing said piezo-electric substrate to a predetermined thickness; and spraying, with said diaphragm as a protective substrate, fine particles onto a piezo-electric substrate portion which does not intersect said diaphragm portion, for removing.
Yet another aspect of the present invention is
a manufacturing method for a liquid injection device in which a liquid pressurizing chamber and a liquid pressurizing member are separated at a gap with a predetermined size therebetween, and the interval between said liquid pressurizing chamber and said liquid pressurizing member is controlled to thereby inject said liquid, comprising the steps of: superimposing a major substrate constituting said liquid pressurizing member on a major substrate constituting said liquid pressurizing chamber; and fixing at least said major substrate constituting said liquid pressurizing member and said major substrate constituting said liquid pressurizing chamber through the use of a member such as spring material or screw material without the aid of any adhesive.
The present invention described above enables dot modulation with wide modulation width, and implements more colorful color printing with multi-tone.
Also, according to a manufacturing method for a liquid injection device of the present invention, it becomes possible to easily manufacture a liquid injection device with wide modulation width at low cost.